ABSTRACT This project explores the untested hypothesis that consumption of soy-based formula during infancy is associated with increased seizure incidence in fragile X syndrome (FXS). FXS is the most common form of inherited intellectual disability. Serendipitous findings from our laboratory indicate that soy-based diets are associated with increased seizure incidence in a mouse model of FXS, Fmr1KO mice, as well as in infants later diagnosed with autism. This grant application is a pilot study to determine if there is an association between soy-based infant formula consumption and seizure history in FXS subjects enrolled in the Fragile X Online Registry with Accessible Research Database (FORWARD), the largest registry of FXS patients. Positive results will support prospective evaluation to determine if avoidance of soy-based infant formula in FXS babies mitigates disease phenotypes including seizures. Newborn screening would be required for an early FXS diagnosis. The long-term goals of this research are to understand how consumption of a soy-based diet during infancy affects childhood development in FXS.